I wouldn't expect a reduction, if i i had a show the next day or wanted mine in due to the heat then I am free to use my stable as I please.
I did have one yard though that had fixed DIY rates for summer (may-end of sept) and winter (oct to end of April). In the winter we were charged...
I think it depends, I brought my horse from the first stables we were at which was a working farm with daily but limited individual winter turnout, excellent hacking on the doorstep. That worked for a while as we had no school and I was happy to just ride at the weekends during the winter...